1. Field of the Invention
The present invention relates to a nitride semiconductor laser device, and to a method for fabricating a nitride semiconductor laser device. More particularly, the present invention relates to a nitride semiconductor laser device that uses as the substrate thereof a nitride semiconductor.
2. Description of Related Art
One feature of nitride semiconductors, for example GaN, AlGaN, GaInN, and AlGaInN, is that they have higher band gap energies than AlGaInAs-based and AlGaInP-based semiconductors. Another feature of such nitride semiconductors is that they are direct-transition semiconductor materials.
Having these features, nitride semiconductors have recently been receiving much attention as materials for building semiconductor light-emitting devices, such as semiconductor lasers that emit light in a short-wavelength region ranging from ultraviolet (blue) to green and light-emitting diodes that emit light in a wide wavelength range covering from ultraviolet (blue) to red. With this trend, various applications of nitride semiconductors have been researched and developed in the fields of high-density optical disks, full-color displays, environmental and medical equipment, and many other fields.
Nitride semiconductors have also been arousing expectations as materials for building high-output high-frequency electronic devices that can operate at high temperatures. Moreover, nitride semiconductors have higher thermal conductivity than GaAs-based or other semiconductors, and are thus expected to find applications in devices that operate at high temperatures and at high outputs. Furthermore, nitride semiconductors do not require materials comparable with arsenic (As) used in AlGaAs-based semiconductors or cadmium (Cd) used in ZnCdSSe-based semiconductors, or materials from which such materials are obtained, such as arsine (AsH3). Thus, nitride semiconductors are also expected as compound semiconductor materials that are environment-friendly.
However, conventionally, the fabrication of nitride semiconductors suffers from extremely low yields, meaning that, relative to the total number of nitride semiconductor laser devices fabricated on a single wafer, the number of usable ones is very small. One reason for low yields is the development of cracks in the nitride semiconductor growth layer. Cracks may develop both from faults in the substrate and from faults in the nitride semiconductor growth layer laid on top of the substrate.
Theoretically, it is desirable that a nitride semiconductor growth layer, such as one formed from GaN, be grown and formed on a GaN substrate. To date, however, there has been developed no high-quality GaN single crystal substrate of which the lattice matches with that of GaN. For this reason, as substrates of which the lattice constant differs comparatively little from that of GaN, SiC substrates are occasionally used instead.
However, SiC substrates are expensive, are difficult to form in large diameters, and are liable to produce tensile strains, with the result that they are more liable to develop cracks. In addition, any material for the substrate of a nitride semiconductor is required to withstand a growth temperature as high as about 1,000° C. and be resistant to discoloration and corrosion in the atmosphere of ammonia gas, which is the material for GaN.
For the reasons discussed above, it is sapphire substrates that are typically used as substrates on top of which to lay a nitride semiconductor growth layer. However, a sapphire substrate exhibits comparatively severe lattice mismatch (about 13%). To overcome this, on top of a sapphire substrate, a buffer layer formed from GaN, AlN, or the like is formed by low-temperature growth, and then, on top of the buffer layer, a nitride semiconductor growth layer is grown. Even this cannot completely eliminate strains, with the result that cracks still develop depending on the composition and film thickness of the growth layer and other conditions.
To overcome this, according to one conventionally proposed method for fabricating a nitride semiconductor device using a GaN substrate, a nitride semiconductor laser device is produced by using a GaN substrate that has previously been so processed as to minimize the effects of such regions thereon as exhibit poor crystallinity (Japanese Patent Application Laid-Open as No. 2003-124573 on Apr. 25, 2003, hereinafter referred to as Patent Publication 1).
However, it is not only from faults in the substrate that cracks develop. When a nitride semiconductor laser device is produced, a nitride semiconductor growth layer is laid on top of a substrate. Here, the nitride semiconductor growth layer is composed of different kinds of film, such as GaN, AlGaN, InGaN, etc. Since these individual films of which the nitride semiconductor growth layer is composed have different lattice constants, lattice mismatch arises, resulting in the development of cracks.
To overcome this, according to another conventionally proposed method, after the growth of a nitride semiconductor growth layer, depressions are formed on the surface thereof, without the surface being made flat. This helps reduce cracks (Japanese Patent Application Laid-Open as No. 2002-246698 on Aug. 30, 2002, hereinafter referred to as Patent Publication 2).
By this method, it is possible to reduce both cracks that develop from faults in the substrate and cracks that develop from lattice mismatch between the individual films of which the nitride semiconductor growth layer formed on top of the substrate is composed.
In a case where, as described above, a nitride semiconductor laser device is produced by using a previously processed substrate, the nitride semiconductor growth layer thereof is structured as shown in FIG. 7.
Specifically, on top of the etched surface of an n-type GaN substrate 60 (see FIGS. 6A and 6B), a nitride semiconductor growth layer 11 is formed as described below.
For example, on top of the n-type GaN substrate 60, the following layers are laid on top of one another in the order named: a 2.0 μm thick n-type GaN layer 70; a 1.5 μm thick n-type Al0.062Ga0.938N first clad layer 71; a 0.2 μm thick n-type Al0.1Ga0.9N second clad layer 72; a 0.1 μm thick n-type Al0.062Ga0.938N third clad layer 73; a 0.1 μm thick n-type GaN guide layer 74; a multiple quantum well active layer 75 composed of three pairs of a 4 nm thick InGaN layer and a 8 nm thick GaN layer laid on top of one another; a 20 nm thick p-type Al0.3Ga0.7N evaporation prevention layer 76; a 0.08 μm thick p-type GaN guide layer 77; a 0.5 μm thick p-type Al0.062Ga0.938N clad layer 78; and a 0.1 μm thick p-type GaN contact layer 79.
In this way, by laying the nitride semiconductor growth layer 11 on the previously processed n-type GaN substrate 60 by MOCVD (metal organic chemical vapor deposition), a nitride semiconductor wafer having depressions on the surface of the semiconductor growth layer 11 as shown in FIGS. 6A and 6B is produced.
In crystallography, it is customary to add an overscore to the absolute value of the index indicating a plane or orientation of a crystal if the index is negative. However, in the present specification, since such notation is impossible, a negative index will be indicated by placing the minus sign “−” in front of the absolute value thereof.
In the present specification, some terms are used in specific senses. A “trough” denotes a depressed portion formed in the shape of a stripe on the surface of a previously processed substrate as shown in FIGS. 6A and 6B. A “ridge” denotes an elevated portion formed likewise in the shape of a stripe.
A “previously processed substrate” denotes a substrate produced by forming troughs and ridges on the surface of a nitride semiconductor substrate or on the surface of a nitride semiconductor growth layer laid on top of the surface of a nitride semiconductor substrate.
In the n-type GaN substrate 60 shown in FIGS. 6A and 6B, stripe-shaped troughs are formed in the [1-100] direction by a dry etching technique such as RIE (reactive ion etching). These troughs are 5 μm wide, are 3 μm deep, and are formed with a period of 400 μm between adjacent troughs. On top of the so etched n-type GaN substrate 60, the nitride semiconductor growth layer 11, having a multiple-layer structure as shown in FIG. 7, is formed by a growth method such as MOCVD.
However, producing a nitride semiconductor laser device by the technique disclosed in Patent Publication 2 mentioned above, specifically by using a previously processed GaN substrate and epitaxially growing a nitride semiconductor growth layer on top of the substrate by MOCVD or the like, has been confirmed to contribute indeed to the reduction of cracks but not to a satisfactory improvement in yields.
This is because the depressions left on the nitride semiconductor growth layer degrade the flatness of the films of which it is composed. With degraded flatness, the individual layers have thicknesses varying from one place to another within the nitride semiconductor growth layer. This causes the characteristics (such as FFP (far-field pattern), threshold current, and slope) of the produced nitride semiconductor laser devices vary from one device to another. This reduces the number of devices of which the characteristics fall within the desired ranges. Thus, to improve yields, it is necessary not only to reduce the development of cracks but also to improve the flatness of the individual films.
FIG. 8 shows the flatness, as actually measured in the [1-100] direction, of the surface of a nitride semiconductor wafer formed as shown in FIGS. 6A, 6B, and 7. The measurements were taken under the following conditions: measurement length=600 μm; measurement duration=3 s; probe needle pressure=30 mg; and horizontal resolution=1 μm per sample. The graph in FIG. 8 shows that, within the 600 μm wide region in which the measurements were taken, the level difference between the highest and lowest points was 200 nm.
This variation in flatness results from the fact that, as shown in FIG. 6B, the film thicknesses of the individual layers of the nitride semiconductor growth layer 11 laid on top of the surface of the n-type GaN substrate 60 vary from one place to another within the wafer.
Consequently, the characteristic of nitride semiconductor laser devices greatly vary according to where on the surface of a wafer they are produced. Moreover, the thickness of the Mg-doped p-type layer (i.e., the sum of the layer thicknesses from the p-type GaN guide layer 77 through the p-type GaN contact layer 79), which thickness greatly affects the characteristic of nitride semiconductor laser devices, greatly varies according to where on the surface of the substrate it is formed.
In the process of forming a ridge structure as a current-narrowing structure, whereas ridges are left in the shape of 2 μm wide stripes, the rest is etched off by a dry etching technique using an ICP (inducting coupled plasma) machine.
Thus, if the thickness of the p-type layer before etching varies from one place to another within the wafer surface, the film thickness of the p-type layer that remains after etching, which thickness most greatly affects the characteristics of nitride semiconductor laser devices, accordingly varies greatly from one place to another within the wafer surface.
Because of the factors discussed above, the layer thickness varies among individual nitride semiconductor laser devices. In addition, even within a single nitride semiconductor laser device, while the thickness of the remaining p-type layer is almost zero at some places, it is considerably great at other places. This variation in the thickness of the remaining p-type layer greatly affects the characteristics, including the life, of nitride semiconductor laser devices.
Next, using a light interference microscope, the thickness of the p-type layer before a ridge structure was formed by etching was measured. Here, with the design value of the thickness set at 0.700 μm, 20 measurements were taken respectively at different places within the wafer surface, and the mean deviation σ of those measurements were calculated. The mean deviation σ indicates the variation of the film thickness among the 20 measurements thereof. The greater the mean deviation σ, the greater the variation of the various characteristic, such as FFP (far-field pattern), threshold current, and slope efficiency, of nitride semiconductor laser devices.
The mean deviation σ of the thickness of the p-type layer formed on the wafer produced by growing the nitride semiconductor growth layer 11 on top of the conventional n-type GaN substrate 60 as shown in FIGS. 6A and 6B was 0.07. To satisfactorily reduce the variation of the characteristics of nitride semiconductor laser devices, the mean deviation σ needs to be reduced to 0.01 or lower. The mean deviation σ of the thickness of the p-type layer formed on the wafer produced by growing the nitride semiconductor growth layer 11 shown in FIGS. 6A and 6B, however, does not meet this requirement. Incidentally, the mean deviation is calculated by adding together the differences of the individual values of the 20 measurements of the layer thickness from the mean value of the 20 measurements and then dividing the result by 20.
This large variation in layer thickness within the wafer surface is considered to result from the fact that, when the films are epitaxially grown in the ridge portions of the previously processed substrate, their growth speed is affected by the troughs, resulting in uneven growth.
Specifically, as shown in FIG. 9A, on the n-type GaN substrate 60 having troughs formed thereon, as epitaxial growth progresses, a top growth portion 90, a side growth portion 91, and a bottom growth portion 92 grow in an uncarved region 93, in the side face 94 of a carved region, and in the bottom face 95 of the carved region, respectively.
When a semiconductor thin film is grown in this way, the side growth portion 91, indicated with hatching in FIG. 9A, greatly affects the flatness of the top growth portion 90. As shown in FIG. 9A, let the film thickness of the side growth portion 91 be X.
It has been confirmed that, as the growth of the semiconductor thin film in the side growth portion 91 progresses as shown in FIG. 9B, the growth speed of the semiconductor thin film in the top growth portion 90 is affected to vary.
Specifically, the larger the film thickness X of the side growth portion 91, the lower the growth speed of the semiconductor thin film on the top growth portion 90, and thus the smaller the film thickness on the top growth portion 90. By contrast, the smaller the film thickness X of the side growth portion 91, the higher the growth speed of the semiconductor thin film on the top growth portion 90, and thus the greater the film thickness on the top growth portion 90. Thus, the film thickness of the semiconductor thin film on the surface of the top growth portion 90 varies greatly according to the film thickness X of the side growth portion 91.
The film thickness X of the side growth portion 91 varies from one place to another in the [1-100] direction because of the variation of the off angle within the surface, unevenness in the substrate itself such as the variation of the curvature thereof within the surface, unevenness of the epitaxial growth speed within the substrate surface, unevenness of the carving process within the substrate surface, and other factors. As a result, as discussed above, the flatness, within the wafer surface, of the semiconductor thin film laid on the surface of the top growth portion 90 is degraded.
Moreover, the greater the film thickness X of the side growth portion 91, the greater the variation, within the substrate surface, of the film thickness X of the side growth portion 91, and thus the more the flatness within the wafer surface is degraded. Thus, to obtain good flatness, the film thickness X of the top growth portion 90 needs to be reduced.
Moreover, the semiconductor thin film in the side growth portion 91 not only epitaxially grows directly on the side face, but its growth is also promoted by “creep-up growth,” whereby the semiconductor thin film grown in the bottom growth portion 92 creeps up to the side growth portion 91.
FIG. 10 is a conceptual diagram illustrating how creep-up growth occurs from the bottom growth portion 92 of the carved region to the side growth portion 91. This creep-up growth further increases the film thickness X of the side growth portion 91 (see FIGS. 9A and 9B), and thereby affects the flatness within the wafer surface.